(i) Field of the Invention
The present invention relates to expandable styrene resin particles and a production process thereof, expandable styrene beads, and a foamed article.
(ii) Description of the Related Art
An expandable styrene resin is used for a number of food containers, packing materials, cushioning materials and others by taking advantage of its excellent heat insulation properties, economy and sanitation. A foamed article thereof is produced by heating particles of the expandable styrene resin by steam or the like to be pre-foamed to a desired bulk density, subjecting the particles to an aging step, and filling the resulting particles into a mold to be foamed under heating again. The expandable styrene resin particles are required to have excellent expandability in the pre-expanding step or the expand-molding step in the mold under heating and to have high strength or an excellent appearance when molded into an article.
Heretofore, to enhance the strength of the foamed article, some methods has been investigated, for example, increasing of the density of a molded article, or increasing of the thickness of the molded article itself. However, any of these methods can be hardly considered to be economical since the weight of the molded article increases and also have problems from an environmental standpoint. As for the expandable styrene resin particles, some methods has been investigated, for example, increasing the molecular weight, or controlling or reducing a kind or an amount of plasticizer for plasticizing the resin particles.
On the other hand, to improve the expandability of the expandable styrene resin particles, a method of decreasing the molecular weight, a method of plasticizing the resin particles, a method of changing a composition ratio of expanding agents and other methods have been investigated.
Such methods of enhancing the strength of the foamed article and such methods of improving the expandability are generally contradictory to each other, and hence, it has been difficult to simultaneously satisfy both the methods.
For solving these problems, resin particles, whose outermost layer portion and central portion have a low molecular weight and middle portion has a high molecular weight, are proposed in Japanese Patent Laid-Open No. 295756/1996.
However, the above particles have a problem that since the molecular weight of the outermost layer portion is low like the central portion, thermal fusion is promoted excessively at the time of heat-expand-molding, thereby degrading a surface finish of a molded article.
Further, in Japanese Patent Laid-Open No. 188454/1995, resin particles whose surface layer portion has a higher molecular weight than that of the whole particle are disclosed.
It is described in this publication that when the surface layer portion has an excessively high molecular weight, the expandability deteriorates and the appearance of the resulting molded article is degraded with a lower strength. This is assumed to be because the molecular weight of the central portion of the particles disclosed therein also becomes high as the molecular weight of the surface layer portion becomes high. That is, the above resin particles have a problem that the molecular weight of the surface layers cannot be sufficiently increased.
Further, heretofore, to obtain the foamed articles having a good appearance, it was essential to eliminate gaps among beads completely when styrene beads filled in a mold are foamed under heating. However, it was difficult to eliminate all gaps among the beads. Under the circumstances, to eliminate as many voids as possible, the properties of expandable styrene resin particles themselves and molding techniques including the functions of molding machines and the like have been improved.
For example, to improve the properties of the expandable styrene resin particles themselves, efforts were made to control the type and amount of plasticizer for plasticizing the resin particles, or to render the molecular weight lower. However, these methods lead to a reduction in the heat resistance of the resin particles and have a problem that the surface of a molded article is molten by heat-expanding at the time of molding, thereby increasing voids.
Further, to improve molding techniques including the functions of molding machines and the like, the following have been studied. As for the molding machines, a control method in a heating step has been studied, and a method of using steam more efficiently was employed. As for molds, a method of using steam more effectively by, for instance, increasing the number of slits has been studied. However, these methods have problems that additional costs including costs for improving the machines and the molds are so large and it is difficult to improve all of these at once.